Check Your Understanding 249
11. Which of the following media is used to interconnect the ISDN BRI port to the
service-provider device?
A. CAT 5 UTP straight-through
B. CAT 5 UTP crossover
C. Coaxial
D. Fiber optic
12. What type of connector is used for DSL connection?
A. RJ-45
B. RJ-11
C. F
D. DB-9
13. What type of connector is used to connect a router and a cable system?
A. RJ-45
B. RJ-11
C. F
D. AUI
14. What type of cable is used to connect a terminal and a console port?
A. Straight-through
B. Rollover
C. Crossover
D. Coaxial
chpt_04.fm Page 249 Tuesday, May 27, 2003 9:01 AM
Objectives
Upon completion of this chapter, you will be able to
■ Describe briefly the history of Ethernet
■ Identify the IEEE standards
■ Understand MAC addressing
■ Identify the common fields of the data link layer frames
■ Describe Media Access Control (MAC)
■ Describe CSMA/CD

■ Understand the operation of Ethernet
■ Identify different types of collisions
■ Explain collisions, collision domains, and broadcast domains
■ Identify the Layer 1, 2, and 3 devices used to create collision domains and
broadcast domains
■ Discuss data flow and the problem with broadcasts
■ Understand collisions and collision domains
■ Understand broadcasts and broadcast domains
■ Understand segmentation of a network and the devices used to create the
segments
1102.book Page 250 Tuesday, May 20, 2003 2:53 PM
Chapter 5
Ethernet Fundamentals
Ethernet, in its various forms, is the most widely used local-area network (LAN) tech-
nology. Ethernet was designed to fill the middle ground between long-distance, low-speed
networks and specialized, computer-room networks carrying data at high speeds for very
limited distance.
Ethernet is well suited to applications in which a local communication medium must carry
sporadic, occasionally heavy traffic at high-pack data rates. It was designed to enable
sharing resources on a local workgroup level. Design goals include simplicity, low cost,
compatibility, fairness, low delay, and high speed.
In this chapter, you learn about the history of Ethernet and IEEE Ethernet standards. This
chapter discusses the operation of Ethernet, Ethernet framing, and error handling, as well
as the different types of the collisions on Ethernet networks. In addition, this chapter
introduces collision domains and broadcast domains. Finally, this chapter describes seg-
mentation and the devices used to create network segments.
Please be sure to look at this chapter’s associated e-Lab Activities, Videos, and Photo-
Zooms that you will find on the CD-ROM accompanying this book. These CD elements
are designed to supplement the material and reinforce the concepts introduced in this
chapter.

History and Evolution of Ethernet
LANs are high-speed, low-error data networks that cover a relatively small geographic
area (up to a few thousand meters). LANs connect workstations, peripherals, terminals,
and other devices in a single building or other geographically limited area.
1102.book Page 251 Tuesday, May 20, 2003 2:53 PM
252 Chapter 5: Ethernet Fundamentals
Ethernet is the dominant LAN technology in the world. Most of the traffic on the Inter-
net originates and ends with an Ethernet connection. From its beginning in the 1970s,
Ethernet has evolved to meet the increasing demand for high-speed LANs. When a
new medium, fiber optics, was produced, Ethernet adapted to take advantage of fiber’s
great bandwidth and low error rate. Now the same basic protocol that transported data
at 3 megabits per second (Mbps) in 1973 is carrying data at 10 gigabits per second (Gbps).
Ethernet’s success is a result of its simplicity and ease of maintenance, its capability to
incorporate new technologies, its reliability, and its low cost of installation and upgrade.
With the introduction of Gigabit Ethernet, what started as a LAN technology has now
had its reach extended to distances that make Ethernet a metropolitan-area and even a
wide-area networking standard. This section provides an overview of the Ethernet,
including the history of Ethernet, Ethernet naming convention, and Ethernet frame
formats.
Introduction to Ethernet
The original idea for Ethernet grew out of the problem of allowing two or more users
to use the same medium without each user’s signals interfering with each other. This
problem of multiple user access to a shared medium was studied in the early 1970s
at the University of Hawaii. A system called Alohanet was developed to allow various
stations on the Hawaiian Islands to each have structured access to the shared radio fre-
quency band in the atmosphere. The original technology that today’s Ethernet is based
on was wireless. This work later formed the basis for the famous Ethernet MAC method
known as carrier sense multiple access collision detect (CSMA/CD). CSMA/CD is dis-
cussed in more detail later in this chapter.
The original version of Ethernet was the world’s first LAN. It was designed more than

30 years ago by Robert Metcalfe and his coworkers at Xerox. The first Ethernet standard
was published by a consortium of Digital Equipment Company, Intel, and Xerox (DIX)
in 1980. Metcalfe wanted Ethernet to be a shared standard from which everyone could
benefit. Therefore, DIX made the new standard an open standard, meaning that it was
available to any company. This was not often done in the computer industry. The first
products developed using the Ethernet standard were sold during the early 1980s. Ether-
net products transmitted at 10 Mbps over thick (about the diameter of your smallest
finger) coaxial cable up to a distance of 2 km. Ethernet was an instant success.
The Institute of Electrical and Electronic Engineers (IEEE) is a professional organiza-
tion that defines network standards. In 1985, the IEEE standards committee for local
and metropolitan networks published its standards for LANs. The IEEE LAN standards
are the predominant and best-known LAN standards in the world today. These stan-
dards start with the number 802. The standard that was based on Ethernet is standard
1102.book Page 252 Tuesday, May 20, 2003 2:53 PM
History and Evolution of Ethernet 253
802.3. The IEEE wanted to make sure that its standards were compatible with and fit
into the ISO’s OSI reference model. The IEEE divides the OSI data link layer into two
separate sublayers: Media Access Control (MAC) and Logical Link Control (LLC).
As a result, some small modifications to the original Ethernet standard were made in
the 802.3 standard. Some differences exist between the DIX Ethernet and the 802.3
specifications. However, the differences between the two standards are so minor that
any Ethernet network interface card (NIC) can transmit and receive Ethernet and 802.3
packets and frames. Essentially, Ethernet and IEEE 802.3 are the same standards. Just
remember that 802.3 is now the official IEEE Ethernet standard.
During the mid-1980s, Ethernet’s 10-Mbps bandwidth was more than enough for
the PCs of that era. By the early 1990s, PCs had become much faster and people were
beginning to complain about the bottleneck caused by the small bandwidth of Ether-
net LANs. In 1995, the IEEE announced a standard for a 100-Mbps Ethernet. This
was followed by standards for Gigabit (1 billion bits per second) Ethernet in 1998 and
1999. IEEE approved the standards for 10-Gb Ethernet in June 2002. These more

modern standards are still Ethernet (802.3).
All the new Ethernet standards are essentially compatible with the original Ethernet
standard. An Ethernet packet (a frame) could leave an older 10-Mbps NIC in a PC,
eventually be placed by a router onto a 10-Gbps Ethernet fiber link, and then end up
at a 100-Mbps Ethernet card. As long as the packet stayed on Ethernet networks, it
would not be changed. This illustrates one of the main reasons for Ethernet’s great
success—it is very scalable. That means that the bandwidth of the network could be
increased again and again without changing the underlying Ethernet technology.
The original Ethernet (IEEE 802.3) has been supplemented a number of times to incor-
porate new transmission media and to enable higher transmission rates. However, it is
important to understand that the essential qualities of the original Ethernet have been
retained. The Ethernet 802.3 standards all belong to the same family. Differences exist
between the standards, but their similarities are greater than their differences. What
has been retained from the original in each new standard means that the 802.3 family
of protocols are all compatible.
IEEE Ethernet Naming Rules
The term Ethernet refers to a family of networking technologies that include original
Ethernet, Fast Ethernet, Gigabit Ethernet (or Gig-E), and 10-Gb Ethernet (or 10-G).
These various types of Ethernet are discussed in detail in Chapter 6, “Ethernet Tech-
nologies and Ethernet Switching.”
1102.book Page 253 Tuesday, May 20, 2003 2:53 PM
254 Chapter 5: Ethernet Fundamentals
Ethernet interfaces can range from US$10 to $100,000. Ethernet speeds can be 10,
100, 1000, or 10,000 Mbps. This section explores more details of original Ethernet
(10BASE-T), Fast Ethernet (100BASE-TX and 100BASE-FX), Gigabit Ethernet
(1000BASE-T and 1000BASE-X), and 10-Gb Ethernet. Two features of Ethernet
remain consistent across all forms of Ethernet: the basic frame format and the IEEE
sublayers of OSI Layer 2.
When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues
a new supplement to the 802.3 standard. The new supplements are given a one- or

two-letter designation—for example, 802.3u is Fast Ethernet. An abbreviated descrip-
tion (called an identifier) also is assigned to the supplement. The following are examples
of some of the supplements:
■ 10BASE2 (IEEE 802.3a)
■ 10BASE5 (IEEE 802.3)
■ 100BASE-T (IEEE 802.3i)
■ 1000BASE-TX (IEEE 802.3X)
As you can see, the abbreviated description consists of these parts:
■ A number indicating the number of megabits per second transmitted
■ The word BASE, indicating that baseband signaling is used
■ Numbers (the 2 and 5) that refer to the coaxial cable segment length
(the 185m length has been rounded up to 2, for 200)
■ One or more letters of the alphabet indicating the type of medium used
(F = fiber optical cable, T = copper unshielded twisted pair)
Baseband signaling is the simplest method of signaling. In baseband signaling, the whole
bandwidth of the transmission medium is used for the signal. The data signal (a voltage
on UTP or a flash of light on fiber) is transmitted directly over the transmission medium.
No other special signal (known as a carrier signal) is required. Ethernet uses baseband
signaling.
A second method of signaling, broadband signaling, is not used in Ethernet. In broad-
band signaling, the data signal is never placed directly on the transmission medium.
Instead, an analog signal called the carrier signal is modulated by the data signal. Then
this modulated carrier signal is transmitted. Radio broadcasts and cable TV use broad-
band signaling.
1102.book Page 254 Tuesday, May 20, 2003 2:53 PM
History and Evolution of Ethernet 255
Like the International Organization for Standardization (ISO), the IEEE is a standards-
making organization. The manufacturers of networking equipment are not required
to fully comply with all the specifications of any standard. The goals of IEEE are as
follows:

■ To supply the engineering information necessary to build devices that comply
with an Ethernet standard
■ To not stifle innovation by manufacturers
If you are setting up or maintaining a small LAN, you can buy all your equipment
from one reputable manufacturer. Then you can be assured of the compatibility of
all the devices. However, if you are responsible for a large network made up of many
smaller Ethernet LANs with equipment from a mix of vendors, your situation is very
different. The capability to use equipment from a variety of vendors to interoperate
reliably is a very important issue. It is unfortunate that no industry or governmental
agency exists to test and certify that a device fully meets an IEEE standard. However,
that is a very good reason for you to educate yourself about the standards and the
industry. The fact is, you have only your own knowledge and that of your coworkers
on which to base your design and purchasing decisions.
IEEE 802.3/Ethernet and the OSI Model
LAN standards define the physical media and the connectors used to connect devices
to media at the physical layer of the OSI reference model. LAN standards also define
the way devices communicate at the data link layer. In addition, LAN standards define
how to encapsulate protocol-specific traffic in such as way that traffic going to differ-
ent upper-layer protocols can use the same channel that passes though that layers of
the OSI model. To provide these functions, the IEEE Ethernet data link layer has two
sublayers:
■ Media Access Control (MAC) (802.3)—As the name implies, the MAC sublayer
defines how to transmit frames on the physical wire. It handles physical address-
ing associated with each device, network topology definition, and line discipline.
■ Logical Link Control (LLC) (802.2)—As the name implies, the LLC sublayer is
responsible for logically identifying different protocol types and then encapsulat-
ing them. A type code or a service access point (SAP) identifier performs the logical
identification. The type of LLC frame used by an end station depends on what
identifier the upper-layer protocol (such as IP) expects. Although IEEE 802.2
represents one standard type of frame encapsulation, there are others, such as

Ethernet II (used primarily with TCP/IP–based Ethernet LANS. These are dis-
cussed later in the chapter.
1102.book Page 255 Tuesday, May 20, 2003 2:53 PM
256 Chapter 5: Ethernet Fundamentals
As shown in Figure 5-1, the IEEE 802.3 standard defines the physical layer (Layer 1)
and the MAC portion of the data link layer (Layer 2).
Figure 5-1 802.3 Ethernet and the OSI Model
Figure 5-2 maps a variety of technologies to OSI Layer 1 and the lower half of Layer 2.
In this book, we focus primarily on Ethernet LAN technology. Layer 1, the physical
layer, involves interfacing with media, signals, bit streams that travel on media, com-
ponents that put signals on media, and various topologies. The physical layer performs
a key role in the communication that takes place between computers, but its efforts
alone are not enough. Each of its functions has its limitations. Layer 2 addresses these
limitations.
Figure 5-2 LAN Specifications and OSI Model
1102.book Page 256 Tuesday, May 20, 2003 2:53 PM
History and Evolution of Ethernet 257
For each limitation in Layer 1, Layer 2 has a solution, as documented in Table 5-1.
The Layer 2 sublayers, LLC and MAC, are active, vital agreements that make technol-
ogy compatible and computer communication possible. The MAC sublayer is concerned
with the physical components that will be used to communicate the information. Like
the other layers, the LLC remains relatively independent of the physical equipment that
will be used for the communicative process. The LLC allows multiple Layer 3 protocols,
such as IP and IPX, to be simultaneously supported along with multiple frame types.
Figure 5-3 maps a variety of Ethernet technologies to the lower half of OSI Layer 2,
and all of Layer 1. Although there are other varieties of Ethernet, the ones depicted are
the most widely used and are the focus of this course.
Figure 5-3 Ethernet Technologies and OSI Model
Table 5-1 Layer 1 Limitations Versus Layer 2 Solutions
Layer 1 Limitation Layer 2 Solution

Layer 1 cannot communicate with the
upper-level layers.
Layer 2 communicates with upper-
level layers via the LLC sublayer.
Layer 1 cannot identify computers. Layer 2 identifies computers using the
MAC addressing scheme.
Layer 1 can only describe streams of bits. Layer 2 uses framing to organize or
group the bits. (This process ulti-
mately provides a way for the bits to
convey meaning.)
Layer 1 cannot decide which computer will
transmit binary data from a group that is
all trying to transmit at the same time.
Layer 2 uses the MAC sublayer to
accomplish this.
Logical Link Control Sublayer
802.3 Media Access Control
Physical
Signaling
Layer
Physical
Medium
10BASE5 (500 m)
50 Ohm Coax N-Style
1000BASE-LX (550-5000 m)
MM Fiber Sc
10BASE2 (185 m)
50 Ohm Coax BNC
10BASE-T (100 m)
100 Ohm UTP RJ45

10BASE-TX (100 m)
100 Ohm UTP RJ45
100BASE-FX (228_412 m)
MM Fiber SC
1000BASE-T (100 m)
100 Ohm UTP RJ45
1000BASE-SX (220-550 m)
MM Fiber SC
10BASE-(Various)
MM or Sm Fiber SC
1102.book Page 257 Tuesday, May 20, 2003 2:53 PM
258 Chapter 5: Ethernet Fundamentals
MAC Addressing
To allow for local delivery of frames on the Ethernet, there must be an addressing sys-
tem, a way of naming the computers and interfaces. Every computer has a unique way
of identifying itself. Each computer on a network has a physical address. No two physical
addresses on a network should ever be alike. Referred to as the Media Access Control
(MAC) address, the physical address is located on the NIC. Other terms for the MAC
address include the hardware address, the NIC address, the Layer 2 address, and the
Ethernet address.
Ethernet uses MAC addresses to uniquely identify individual devices. Every device
(PC, router, switch) with an Ethernet interface to the LAN must have a MAC address;
otherwise, other devices cannot communicate with it. A MAC address is 48 bits in
length and is expressed as 12 hexadecimal digits. The first six hexadecimal digits, which
are administered by the IEEE, identify the manufacturer or vendor and thus comprise
the
organizationally unique identifier (OUI). The remaining six hexadecimal digits
comprise the interface serial number, or another value administered by the specific vendor.
MAC addresses sometimes are referred to as burned-in addresses (BIAs) because they
are burned into read-only memory (ROM) and are copied into random-access memory

(RAM) when the NIC initializes. Figure 5-4 illustrates the MAC address format.
Figure 5-4 MAC Address Format
Without MAC addresses, the LAN would be a group of computers without identifiers,
and it would be impossible to deliver an Ethernet frame. Therefore, at the data link
layer, a header and a trailer are added to upper-layer data. The header and trailer con-
tain control information intended for the data link layer entity in the destination sys-
tem. Data from upper-layer entities is encapsulated in the data link layer header and
trailer.
Ethernet and 802.3 LANs are broadcast networks. All stations see all frames. Each station
must examine every frame to determine whether that station is the desired destination.
On an Ethernet network, when one device wants to send data to another device, it can
open a communication pathway to the other device by using its MAC address. When a
Organizational
Unique
Identifier
(OUI)
Vendor Assigned
(NIC Cards, Interfaces)
24 Bits
6 Hex Digits
00 60 2F
Cisco
24 Bits
6 Hex Digits
3A 07 BC
Particular Device
1102.book Page 258 Tuesday, May 20, 2003 2:53 PM